A number of structural classes of compounds with antibacterial properties are known. Historically, the most important classes of antibacterials have been the .beta.-lactams, macrolides, lincosamides, aminoglycosides, tetracyclines, polypeptides, and sulfonamides. The bulk of these antibacterial compounds were isolated originally from molds, fungi or bacteria; synthetic and semi-synthetic compounds, additionally, have proven to be efficacious in the treatment of bacterial infections. In the broadest possible sense, known antibacterials work by influencing at least one of the following cellular processes or characteristics: cell wall synthesis; protein synthesis; nucleic acid synthesis; cellular metabolism; and cytoplasmic membrane permeability. Brief descriptions follow of the mechanisms of action of members of each of the aforementioned classes of antibacterials.
The .beta.-lactam antibiotics inhibit penicillin binding proteins (PBPs). The PBPs are ubiquitous bacterial enzymes that are involved in cell wall biosynthesis (reviewed in Waxman et al., 1983 Annual Review of Biochemistry 58:825-869; Georgopapadkou et al., 1983 Handbook of Experimental Pharmacology 67:1-77; and Ghuysen, 1991 Annual Review of Microbiology 45:37-67); inhibition of these proteins disrupts the biosynthesis of the bacterial cell wall. Specifically, these compounds act as substrate analogs for the PBPs and form an acyl enzyme intermediate. This acyl enzyme intermediate is resistant to subsequent hydrolysis and ties up the enzyme in a relatively long-lived inactive form. Bacteria have responded to the widespread use of .beta.-lactam antibiotics by evolving a class of .beta.-lactam hydrolyzing enzymes known as .beta.-lactamases. These enzymes are one of the sources of drug resistance now being observed in a number of bacterial diseases including tuberculosis, malaria, pneumonia, meningitis, dysentery, bacteremia, and various venereal diseases.
The macrolides are a family of antibiotics whose structures contain large lactone rings linked through glycoside bonds with amino sugars. The most important members of the group are erythromycin and oleandomycin. Erythromycin is active against most Gram-positive bacteria, Neisseria, Legionella and Haemophilus, but not against the Enterobacteriaceae.
Macrolides inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit. Binding inhibits elongation of the protein by peptidyl transferase or prevents translocation of the ribosome or both. Macrolides are bacteriostatic for most bacteria but are bactericidal for a few Gram-positive bacteria.
The lincosamides are sulfur-containing antibiotics isolated from Streptomyces lincolnensis. There are two important lincosamides: lincomycin and clindamycin. Clindamycin is preferred over lincomycin due to its greater potency, fewer adverse side effects, and its more favorable pharmacokinetic properties. Bacterial resistance and cross resistance to clindamycin have begun to emerge. The lincosamides are active against Gram-positive bacteria especially cocci, but also non-spore forming anaerobic bacteria, Actinomycetes, Mycoplasm and some Plasmodium. The lincosamides bind to the 50S ribosomal subunit and thereby inhibit protein synthesis. These drugs may be bacteriostatic or bactericidal depending upon several factors, including their local concentration.
Aminoglycosides are important antibacterials used primarily to treat infections caused by susceptible aerobic Gram-negative bacteria. Unfortunately, they have a narrow margin of safety, producing characteristic lesions in kidney, cochlea, and vestibular apparatus within the therapeutic dose range. Because they are polycations, the aminoglycosides cross cellular membranes very poorly.
The tetracyclines consist of eight related antibiotics which are all natural products of Streptotmyces, although some can now be produced semi-synthetically. Tetracycline, chlortetracycline and doxycycline are the best known members of this class. The tetracyclines are broad-spectrum antibiotics with a wide range of activity against both Gram-positive and Gram-negative bacteria. The tetracyclines act by blocking the binding of aminoacyl tRNA to the A site on the ribosome. Tetracyclines inhibit protein synthesis on isolated 70S or 80S (eukaryotic) ribosomes, and in both cases, their effect is on the small ribosomal subunit. Most bacteria possess an active transport system for tetracycline that will allow intracellular accumulation of the antibiotic at concentrations 50 times as great as that in the surrounding medium. This system greatly enhances the antibacterial effectiveness of tetracycline and accounts for its specificity of action, since an effective concentration is not accumulated in host cells. Thus a blood level of tetracycline which is harmless to mammalian tissues can halt protein synthesis in invading bacteria. The tetracyclines have a remarkably low toxicity and minimal side effects in mammals. The combination of their broad spectrum and low toxicity has led to their overuse and misuse by the medical community and the wide-spread development of resistance has reduced their effectiveness. Nonetheless, tetracyclines still have some important uses, such as in the treatment of Lyme disease.
The polypeptide antibacterials have in common their basic structural elements--amino acids. Representatives of this class include vancomycin, and bacitracin. Vancomycin can be used to treat both systemic and gastrointestinal infections, whereas because of serious systemic toxicities bacitracin. is limited to topical applications. Vancomycin inhibits bacterial cell wall synthesis by inhibiting peptidoglycan synthase, apparently by binding to D-alanyl-D-alanine, a component of the cross-link between chains. This action inhibits peptidoglycan chain elongation, and as might be expected, the effect is bactericidal for most organisms if they are dividing rapidly. Because it does not target penicillin-binding enzymes, vancomycin is not cross-resistant with the .beta.-lactams. Bacitracin is a narrow spectrum antibiotic which inhibits cell wall biosynthesis by inhibiting lipid pyrophosphatase; this enzyme is involved in transmembrane transport of peptidoglycan precursors.
The sulphonamides are usually bacteriostatic and arrest cell growth by inhibiting bacterial folic acid synthesis. They are effective against sensitive strains of Gram-negative and Gram-positive bacteria, Actinomyces, Nocardia and Plasmodia. However, extensive clinical use of sulfonamides over many years has resulted in a high level of resistance and their current use is limited.
Additionally, there are miscellaneous antibacterials that do not fit readily into the structural classes outlined above. A comprehensive discussion of these miscellaneous antibacterials is not warranted; a small number of antibacterials in this group, however, are relevant to the subject compounds. First, U.S. Pat. No. 3,799,929 "Cinchoninic Acid Derivatives", granted to Eli Lilly and Company on Mar. 26, 1974, and 3,870,712 "Cinchoninic Acid Derivatives", granted to Eli Lilly and Company on Mar. 11, 1975, are directed to sets of substituted quinolines represented by structure A. Of particular relevance to the subject compounds, Lilly claims compounds represented by A wherein: 1) R represents 3-indolyl or 1-methyl-3-indolyl; and 2) R.sub.1 represents --OH or lower alkoxy of 1 to 3 carbons.
Moreover, a few quinoline-indole compounds have been found to display biological activity other than against bacteria. Published PCT applications WO 95/32948 "Quinoline Derivatives as Tachykinin NK.sub.3 Receptor Antagonists", and WO 96/02509 "Quinoline Derivatives as NK.sub.3 Antagonists", filed by SmithKline Beecham disclose substituted quinolines represented by structures B and C, respectively. In WO 95/32948, SmithKline Beecham claims compounds represented by B wherein: 1) R.sub.5 is branched or linear C.sub.1-6 alkyl, C.sub.3-7 cycloalkyl, C.sub.4-7 cycloalkylalkyl, optionally substituted aryl, or an optionally substituted single or fused ring heterocyclic group, having aromatic character, containing from 5 to 12 ring atoms and comprising up to four hetero-atoms in the or each ring selected from S, O, N; and 2) X is O, S, or N--CN. In WO 96/02509, SmithKline Beecham claims compounds represented by C wherein: 1) Ar is an optionally substituted phenyl or naphthyl group or an optionally substituted single or fused ring heterocyclic group, having aromatic character, containing from 5 to 12 ring atoms and comprising up to four hetero-atoms in the or each ring selected from S, O, N; and 2) X is O, S, H.sub.2 or N--CN. ##STR1##
Antibacterial resistance is a global clinical and public health problem that has emerged with alarming rapidity in recent years and undoubtedly will increase in the near future. Resistance is a problem in the community as well as in health care settings, where transmission of bacteria is greatly amplified. Because multiple drug resistance is a growing problem, physicians are now confronted with infections for which there is no effective therapy. The morbidity, mortality, and financial costs of such infections pose an increasing burden for health care systems worldwide, but especially in countries with limited resources. Strategies to address these issues emphasize enhanced surveillance of drug resistance, increased monitoring and improved usage of antimicrobial drugs, professional and public education, development of new drugs, and assessment of alternative therapeutic modalities.